[R] Summing up matrices in a list

[Gabor Grothendieck]

Vicky Landsman <msvika <at> mscc.huji.ac.il> writes:

: 
: Dear all,
: I think that my question is very simple but I failed to solve it. 
: I have a list which elements are matrices like this:
: 
: >mylist  
: [[1]]
:      [,1] [,2] [,3]
: [1,]    1    3    5
: [2,]    2    4    6
: 
: [[2]]
:      [,1] [,2] [,3]
: [1,]    7    9   11
: [2,]    8   10   12
: 
: I'd like to create a matrix M<-mylist[[1]]+mylist[[2]]
:      [,1] [,2] [,3]
: [1,]    8   12   16
: [2,]   10   14   18
: 
: Is there a way to create M without looping? 
: Thanks a lot, 
: Vicky Landsman. 

If there are n3 matrices each of dimension n1 x n2 then we can 
create an n1 x n2 x n3 array and apply sum over it:

# get dimensions
n1xn2 <- dim(mylist[[1]])
n3 <- length(mylist)

# create 3d array and sum
arr <- array(unlist(mylist), c(n1xn2, n3))
apply(arr, 1:2, sum)